Foamed products containing endothermic blowing agents and processes

ABSTRACT

A stabilized powdered endothermic blowing agent which is a mixture of a polycarboxylic acid and an inorganic carbonate, the polycarboxylic acid and inorganic carbonate each having been surface treated with a component which prevents water from being absorbed thereby The blowing agent is incorporated into a polymeric concentrate used for producing foamed products.

This application is a division of application Ser. No. 07/688,227, filedApr. 22, 1991; now U.S. Pat. No. 5,106,534, which is a division of Ser.No. 07/352,307, filed May 16, 1989, now U.S. Pat. No. 5,009,810.

TECHNICAL FIELD

The present invention relates to endothermic blowing agents for use inmaking foamed products such as cellular structured plastics or rubbers.In particular, the present invention relates to endothermic blowingagents which have been chemically stabilized to prevent prematurereaction with water.

BACKGROUND ART

The term "foaming agent" or "blowing agent" is used to describe anysubstance which alone or in combination with other substances is capableof producing a cellular structure in a plastic or rubber mass. Thus, theterm includes gases which expand when pressure is released, solublesolids that leave pores when leached out, liquids which develop cellswhen they change to gases, and chemical agents that decompose or reactunder the influence of heat to form a gas.

Known liquid foaming agents include certain aliphatic and halogenatedhydrocarbons, low boiling alcohol, ethers, ketones, and aromatichydrocarbons. Chemical foaming agents range from simple salts such asammonium or sodium bicarbonate to complex nitrogen releasing agents, ofwhich azobisformamide is an important example.

Foaming agents, also known as chemical blowing agents (CBA's) can beutilized in all conventional plastics processes, such as extrusion,calendering, injection molding, coating, expansion casting, androtational molding.

Recognized advantages of foamed plastics include reduction in density,savings in material costs, improved electrical and thermal insulativeproperties, increased strength to weight ratio, and the elimination ofshrinkage, warpage, and sink marks in injection molded parts Foamedplastic products include such diverse items as vinyl flooring, insulatedfood containers, structural foam furniture, business machine housings,simulated leather, and foamed core pipe.

Foaming agents are generally classified as physical or chemical.Chemical foaming agents (generally solids) undergo a chemicaltransformation when producing gas, while physical foaming agents undergoa generally reversible physical change of state. e.g., vaporization.

The two major categories of physical foaming agents include both liquidsand gases. The gas most often is compressed nitrogen. In injectionmolding processes which utilize physical foaming agents, the gas isinjected under high pressure directly into the polymer duringplastication, and the mixed polymer and gas are metered into the mold.When the pressure is relieved, the gas becomes less soluble in thepolymer and expands, forming the cellular structure. Nucleating agents,in the form of finely divided powders and chemical foaming agents,sometimes are used with the gas to yield a finer cell structure.

The preference for nitrogen is due to the fact that nitrogen is inert,nonflammable, leaves no residue in the polymer, and is not temperaturerestrictive. However, the use of processes involving nitrogen is limitedin the industry due to the requirement of licensed specializedequipment. Moreover, the process tends to produce foams with poorerperformance and appearance than those produced by the use of exothermicchemical blowing compounds.

Liquid physical foaming agents include volatile liquids which producegas through vaporization. Common liquid physical foaming agentsgenerally include short-chain aliphatic hydrocarbons (C₅ to C₇) andtheir chlorinated and fluorinated analogs. Liquid physical foamingagents may be used over a wide temperature range in low pressure andatmospheric processes, and are widely used to produce low densitythermoplastics, such as foamed polystyrene, and thermoset polymers, suchas polyesters, epoxy, and polyurethane foam systems.

Chemical foaming agents commonly referred to as blowing agents aregenerally solids that liberate gas(es) by means of a chemical reactionor decomposition when heated. They are necessarily selected for specificapplications or processes based on their decomposition temperatures. Inthis regard, it is important to match the decomposition temperature withthe processing temperature of the polymer to be foamed. If the polymerprocesses at temperatures below that of the chemical foaming agent,little or no foaming will occur. If the process temperature issignificantly above the foaming agent's decomposition temperature, poor(overblown, ruptured) cell structure and surface skin quality willlikely result.

Chemical foaming or blowing agents may be either inorganic or organic.The most common inorganic foaming agent is sodium bicarbonate. Sodiumbicarbonate is inexpensive, nonflammable and begins to decompose at alow temperature; however, it is used only to a very limited extent inthermoplastics. Differential thermal analysis has shown that sodiumbicarbonate decomposes over a broad temperature range and this range isendothermic, contributes to an open cell structure in the finishedproduct, and the released gas (carbon dioxide) diffuses through thepolymer at a much greater rate than nitrogen gas.

Presently used chemical foaming or blowing agents are mostly mixtures ofsodium bicarbonate and sodium hydrogen citrate. The citrate isincorporated together with the sodium bicarbonate in order to facilitatea complete acid assisted decomposition reaction to produce carbondioxide gas. The mixture is also available in a low density polyethyleneconcentrate at various loadings. The mixture is also available as ahydrophobized acid and carbonate which is a free non-dusting powder.

The major advantages associated with utilizing endothermic foaming orblowing agents over their exothermic counterparts include shortdegassing cycles, small cells, smooth surfaces, weight reductions,reduced cycle times, foamed products which have promptly paintablesurfaces, the foaming process is odorless, and the components of thefoaming agents are generally regarded as environmentally safe.

The major disadvantage of existing acid/carbonate systems involves theformation of corrosion on the process equipment. This corrosion isattributed to the action of the citric acid on the lesser grades ofsteel used in some equipment. Another disadvantage associated withexisting acid/carbonate blowing agents is premature reaction with wateror moisture of the blowing agents when they are associated withpolymeric reaction mixtures. This premature reaction when occurringprior to a foaming process detrimentally effects the final products.

Organic foaming or blowing agents can be utilized in most polymerapplications and processes. These compounds release gas (usuallynitrogen and/or ammonia) over a narrow temperature range. The rate ofgas evolution for a given chemical foaming or blowing agent isdetermined by a temperature and time relationship. Applications forchemical foaming agents are generally divided into three areas low,medium and high temperature processing polymers. There are numerousorganic foaming agents available that decompose at various temperatures.

The present invention is an improvement over the prior known chemical,endothermic foaming or blowing agents which provide a highly stableendothermic blowing agent that does not prematurely react with water ormoisture and which reacts in a controlled fashion. Additionally, theblowing agent of the present invention does not contribute to thecorrosion of processing equipment as do prior known chemical,endothermic foaming or blowing agents.

DISCLOSURE OF THE INVENTION

It is accordingly one object of the present invention to provide a novelendothermic blowing agent which has an enhanced stability, release gasin a controlled manner, and is resistant to moisture absorption.

A further object of the present invention is to provide an endothermicblowing agent which does not contribute to the corrosion of processingequipment.

Another object of the present invention is to provide a method forproducing the above endothermic blowing agents.

A still further object of the present invention is to provide for apolymeric concentrate for producing foamed products which incorporatesthe present endothermic blowing agents.

An even further object of the present invention is to provide for foamedproducts which have been produced by a process that utilizes the presentblowing agents.

According to the above objectives, there is provided by the presentinvention an endothermic blowing agents which include a polycarboxylicacid and an inorganic carbonate, each of which has been surface treatedwith a compound which prevents premature reaction due to waterabsorption.

Also provided by the present invention is a polymeric concentrate thatincludes both the present endothermic blowing agents and variouspolymeric reaction mixture.

The present invention further provides for a method to produce the aboveendothermic blowing agents and for foamed products that are produced byprocesses that utilize the endothermic blowing agents.

BEST MODE FOR CARRYING OUT THE INVENTION

The powdered endothermic blowing agent of the present invention isproduced by blending a surface treated polycarboxylic acid and a surfacetreated inorganic carbonate. The surface treatment of these materialscan be performed in a high intensity mixture at a temperature range ofbetween about 72° C. and about 135° C., with a preferred temperaturebeing about 80° C. The preferred level of surface treatment is fromabout 0.1% to about 10%, and most preferably about 3% for thepolycarboxylic acid and about 3% for the inorganic carbonate. Theformation of the surface treatment within this preferred range has beenfound to be effective in avoiding premature reaction resulting fromwater absorption. In surface treating the polycarboxylic acid, a typicallaboratory batch is prepared by adding about 1500 grams ofpolycarboxylic acid, e g., anhydrous citric acid to a 10 liter Henschelmixer or other suitable high intensity mixer. The polycarboxylic acid ismixed at about 1500 rpm and heated until a temperature of about 80° C.is reached. Any heating means such as hot water flowing through a waterjacket surrounding the mixer may be used to attain the desiredtemperature. After reaching the desired temperature about 15.2 grams ofa surface treating compound such as glyceryl monostearate is added tothe heated polycarboxylic acid and mixing is continued at about 1500 rpmand about 80° C. for an additional period of time of approximately fiveminutes. After five minutes the mixing speed is reduced to about 900 rpmand the mixture is allowed to cool below about 60°. Thereafter thesurface treatment of the polycarboxylic acid is complete and the mixtureis unloaded. In surface treating the inorganic carbonate, about 3500grams of an inorganic carbonate, e.g., dry sodium aluminum hydroxycarbonate, is charged into a 10 liter Henschel mixer. The inorganiccarbonate is mixed at a speed of about 1500 rpm and heated by a suitablemeans such as a hot water jacket until a temperature of about 80° C. isreached. After reaching the proper temperature about 108.2 grams of asurface treating compound such as glyceryl monostearate, is added to theinorganic carbonate and mixing is resumed at about 1500 rpm for aboutfive minutes while the temperature is maintained at about 80° C. Afterthe required mixing, the mixing speed is reduced to about 900 rpm andthe mixture is allowed to cool below about 60° C. Thereafter, theinorganic carbonate has been sufficiently surface treated and may beunloaded from the mixer.

In preparing a large scale batch of the above surface treatedcomponents, about 200 pounds of polycarboxylic acid, e.g. citric acid,was charged into a 200 liter Henschel mixer. The polycarboxylic acid wasmixed until a temperature of about 68° C. was reached. In this caseneither steam nor hot water was needed to raise the temperature. Whenthe polycarboxylic acid reached a temperature of about 68° C. two poundsof a surface treating component, e.g., glyceryl monostearate, was addedto the mixer. Mixing continued until the temperature reached about 72°C. after which mixing was continued for an additional period of time ofabout 5 minutes. Thereafter the mixture was allowed to cool to belowabout 60° C. and was unloaded from the mixer. The batch surfacetreatment procedure was repeated for the inorganic carbonate using about150 pounds of dry inorganic carbonate, e.g., sodium aluminum hydroxycarbonate, and about 4.5 pounds of the surface treating component, e.g.,glyceryl monostearate.

The present endothermic blowing agents are prepared by blending togetherthe surface treated polycarboxylic acid and the surface treatedinorganic carbonate. In a preferred embodiment about 1029 grams ofsurface treated inorganic carbonate, e g , sodium aluminum hydroxycarbonate, was placed into a 10 liter Henschel mixer together with about1471 grams of surface treated polycarboxylic acid, e.g., citric acid.The mixture was mixed at about 1500 rpm for a period of time of aboutfive minutes. Thereafter the mixing speed was reduced to about 900 rpmfor a short period of time and unloaded from the mixture.

In the above discussion, the preferred polycarboxylic acid used wascitric acid. However, for purposes of the present invention, othersuitable carboxylic acids include those of the formula: HOOC-RCOOH whereR is an alkylene group of 1 to about 8 carbon atoms which may also besubstituted by one or more hydroxy groups or keto groups and may alsocontain unsaturation. Also included are salts and half salts. Preferredpolycarboxylic acids include citric acid, fumaric acid, tartaric acid,sodium hydrogen citrate and disodium citrate.

The preferred inorganic carbonate utilized in the invention is sodiumaluminum hydroxy carbonate. However, acceptable results are alsoachieved by also using sodium bicarbonate as well as alkali and alkalineearth metal carbonates generally.

In the above examples glyceryl monostearate was utilized as thepreferred surface treatment compound. However, surface treatment can becarried out utilizing any reagent which renders the polycarboxylic acidand the inorganic carbonate hydrophobic. In this regard, suitablesurface treating components found useful for purposes of the presentinvention included various monoglycerides, stearic acid and silanecoupling agents in addition to the above glyceryl monostearate.

In combining the surface treated polycarboxylic acid and the surfacetreated inorganic carbonate to produce the blowing agent the weightratio of these two components needs to be determined based on thedesired structure and composition of the final foam product to beproduced. Typically, the weight ratio of surface treated polycarboxylicacid to surface treated inorganic carbonate may vary from about 0.010 toabout 99.00. However, the weight ratio is preferably maintained betweenabout 1.00 and about 5.00.

The endothermic blowing agent is preferably used by incorporating itinto a polymeric concentrate which is used to produce film products. Inone embodiment according to the present invention the polymeric orplastic concentrates were prepared at loading levels of from about 5% toabout 50% in a single or twin screw extruder utilizing normal techniquesknown to those schooled in the art of plastic compounding. The extrudermust, however, be operated below about 149° C. in order to keep theendothermic blowing agent from foaming prematurely.

In these polymeric or plastic concentrates citric acid may be replacedby other polycarboxylic acids such as fumaric acid, tartaric acid andsodium hydrogen citrate which have higher degradation temperatures andwill allow higher extrusion temperatures. The novel interaction betweenthe polycarboxylic acid and the inorganic carbonate controls thecomposition and the volume of the above gases which serve to foam aplastic for foamed products.

The application areas for these endothermic products include polyvinylchloride, polyolefins, engineering resins, polystyrene, polyphenyleneoxide and other polymetric systems. These foamed products displayexcellent weight reduction and drop weight impact properties and areoutlined in the following examples given by way of illustration only.Other uses for the present novel blowing agent include, for example, itsuse as a propellent generator in aerosol containers.

As described in detail above, the present invention involves anendothermic blowing agent which is a mixture of a polycarboxylic acidand an inorganic carbonate, wherein the polycarboxylic acid and theinorganic carbonate each have been surface treated with a component thatprevents water from being absorbed thereby.

When properly surface treated, the level of surface treatment of thepolycarboxylic acid and the inorganic carbonate is between about 1.0 and5.0 percent by weight. In a preferred embodiment the level of surfacetreatment of said polycarboxylic acid is about 1.0 percent by weight andthe level of surface treatment of said inorganic carbonate is about 3.0percent by weight.

The ratio of the polycarboxylic acid and the inorganic carbonate mayvary over a wide range and will be determined by the final desiredproducts. Generally, the weight ratio of the surface treatedpolycarboxylic acid to the surface treated inorganic carbonate isbetween about 0.10 and about 99.00, with a more desired weight ratiobeing between about 1.00 and about 5.00.

As further discussed in detail above, the present invention alsoinvolves a method of making the endothermic blowing agent. Generally,this method involves the steps of:

(a) heating a dry powder of polycarboxylic acid to a temperature ofbetween about 68° C. and about 80° C. and adding thereto a surfacetreating component to form a surface treated polycarboxylic acid;

(b) heating a dry powder of an inorganic carbonate to a temperature ofbetween about 68° C. and about 80° C. and adding thereto a surfacetreating agent to form a surface treated inorganic carbonate and;

(c) mixing together the surface treated polycarboxylic acid and thesurface treated inorganic carbonate to form the endothermic blowingagent.

In this method, each of the steps is preformed in a high intensity mixerin which the components are agitated,

The endothermic blowing agent is ideally incorporated into a polymericconcentrate that is used for producing foamed products. The polymericconcentrate includes both the endothermic blowing agent and a polymerreaction mixture. Particular polymer reaction mixtures are those thatmay be used to produce polymers of polyvinyl chloride, polyolefins,engineering resins, polypropylene and polystyrene.

Finally, the present invention involves foamed polymeric products thathave been produced by both the above blowing agent and the abovediscussed polymeric concentrate. Such products may be made by processesthat involve either extrusion, calendering, injection molding, coating,expansion casting or rotational molding.

The following examples are presented to illustrate the invention whichis not intended to be considered as being limited thereto. In theexamples and throughout percentages are by weight unless otherwiseindicated.

EXAMPLE I

A 25% concentrate of endothermic blowing agent was prepared inethylene-vinyl acetate using a 30 millimeter twin screw extruder. Theconcentrates were let down to 0.5% active blowing agent inpolypropylene. Plaques (4"×4"×1/4") were then foamed in a 50 ton Newburyinjection molder at 200° C. Table I sets forth the excellent weight lossand impact properties. Comparisons are made with other commerciallyavailable blowing agents.

EXAMPLE II

The procedure used in Example I was repeated, except 1% active blowingagent was used in the final blend of polypropylene. Table II summarizesthe weight reduction property as the weight ratio of inorganic carbonateto polycarboxylic acid varied from 0.25 to 1.30

                  TABLE I                                                         ______________________________________                                        WEIGHT REDUCTION AND IMPACT PROPERTIES                                        OF FOAMED POLYPROPYLENE                                                       (0.5% ACTIVE BLOWING AGENT)                                                                   % WEIGHT   DROP WEIGHT                                        SAMPLE          LOSS       IMPACT (IN/LB)                                     ______________________________________                                        CONTROL (NO     --         43.4                                               BLOWING AGENT)                                                                FCN-20-AZo.sup.1                                                                              4.8        2.5                                                CENBLO MS31N.sup.2                                                                            15.9       5.3                                                HYDROCEROL      14.7       6.0                                                COMPOUND.sup.3                                                                PRESENT INVENTION.sup.4                                                                       16.7       25.0                                               (0.8-ACID: CARBONATE)                                                         PRESENT INVENTION.sup.5                                                                       12.7       27.4                                               (0.6-ACID: CARBONATE)                                                         ______________________________________                                         .sup.1 FCN-20-AZo is an organic azo exothermic type blowing agent             consisting of 1,1',azo bis formamide, produced by Santech, Inc., Toronto,     Ontario, Canada.                                                              .sup.2 CENBLO MS31N is a foaming endothermic type blowing agent consistin     of hydrophobized citric acid and sodium bicarbonate, produced by Central      Chemicals Co., Ltd and sold under the trademark CENBLO.                       .sup.3 HYDROCEROL COMPOUND is a direct gassing foaming type blowing agent     consisting of monosodium citrate and sodium bicarbonate produced by           Boehringer Ingeltheim KG, and sold under the trademark HYDROCEROL             .sup.4 For the purposes of this example of the present invention, citric      acid was the acid and sodium alumino carbonate the carbonate employed.        .sup.5 For the purposes of this example of the present invention, citric      acid was the acid and sodium alumino carbonate was the carbonate employed

                  TABLE II                                                        ______________________________________                                        WEIGHT REDUCTION OF FOAM MOLDED PLAQUES                                       RATIO                                                                         POLYCARBOXYLIC ACID:                                                                              & WEIGHT                                                  INORGANIC CARBONATE)                                                                              REDUCTION                                                 ______________________________________                                        4                   31.6                                                      2                   31.0                                                      1.4                 28.8                                                      1                   24.4                                                      0.8                 22.9                                                      ______________________________________                                    

For the purposes of this example the polycarboxylic acid employed wasalpha-keto-dicarboxylic acid and the inorganic carbonate was sodiumalumino carbonate. The molded plaques comprised polypropylene in whichwas incorporated ethylene-vinyl acetate (EVA), Elvax 250 from DuPont(25% vinyl acetate). The polypropylene was a 25% filled concentrate letdown to polypropylene with 0.75% active blowing agent (Humont 6523).

Although the invention has been described with reference to particularmeans, materials and embodiments, from the foregoing description, oneskilled in the art can ascertain the essential characteristics of thepresent invention and various changes and modifications may be made toadapt the various uses and characteristics thereof without departingfrom the spirit and scope of the present invention as described in theclaims that follow.

What is claimed is:
 1. In a method for the preparation of an endothermicblowing agent which comprises a mixture of a polycarboxylic acid and aninorganic carbonate, wherein said polycarboxylic acid and said inorganiccarbonate are surface treated with a component which prevents water frombeing absorbed by said polycarboxylic acid and said inorganic carbonate,the improvement comprising the steps of:(a) heating a dry powderpolycarboxylic acid to a temperature of between about 68° C. and about80° C. and adding thereto a surface treating component selected from thegroup consisting of mono glycerides, stearic acid, silane couplingagents and mixtures thereof to form a surface treated polycarboxylicacid; (b) heating a dry powder of sodium aluminum hydroxy carbonate to atemperature of between about 68° C. and about 80° C. and adding theretoa surface treating agent selected from the group consisting of monoglycerides, stearic acid, silane coupling agents and mixtures thereof toform a surface treated inorganic carbonate and; (c) mixing together saidsurface treated polycarboxylic acid and said surface treated inorganiccarbonate in a weight ratio of between about 1:1 to 99:1, respectively.2. A method of making an endothermic blowing agent according to claim 1wherein steps (a) through (c) are performed with agitation duringmixing.
 3. A method according to claim 1, wherein said polycarboxylicacid is selected from the group consisting of those of the formulaHOOC-R-COOH and alkali metal salts thereof, wherein R is alkylene of 1to 8 carbon atoms which may contain one or more hydroxy substituents andmay also contain unsaturation.
 4. A method according to claim 3, whereinsaid polycarboxylic acid is critic acid, selected from the groupconsisting of fumaric acid, tartaric acid, sodium hydrogen citrate,monosodium citrate, and mixtures thereof.
 5. A method according to claim1, wherein said surface treating component comprises glycerylmonostearate.
 6. A method according to claim 1, wherein the level ofsurface treatment of said polycarboxylic acid and said inorganiccarbonate is between about 0.1 and 10.0 percent by weight.
 7. A methodaccording to claim 6, wherein said level of surface treatment of saidpolycarboxylic acid is about 1.0 percent by weight.
 8. A methodaccording to claim 6, wherein said level of surface treatment of saidinorganic carbonate is about 3.0 percent by weight.